1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical recording medium drive employing the same.
2. Description of the Background Art
In recent years, an optical pickup apparatus using a holographic optical element as light beam separating means has been studied and developed. FIG. 11 is a diagram showing the construction of a conventional optical pickup apparatus. The optical pickup apparatus is an apparatus for performing focus servo using an astigmatism method and tracking servo using a three-beam method in addition to an information detecting operation, which is disclosed in Japanese Patent Laid-Open No.76035/1991, for example.
As shown in FIG. 11, the optical pickup apparatus comprises a semiconductor laser device 102 for emitting laser light vertically upward, a diffraction grating 103 for dividing the laser light into three light beams, a holographic optical element 104 for transmitting the three light beams obtained by the division and diffracting returned light beams from an optical disk 100, an objective lens 105 for focusing the three light beams transmitted by the holographic optical element 104 on a recording surface of the optical disk 100 as three spots, and an optical detector 106 for detecting the returned light beams diffracted by the holographic optical element 104.
In the above-mentioned optical pickup apparatus, the laser light emitted from the semiconductor laser device 102 is divided into zero order and .+-. 1st order diffracted light beams by the diffraction grating 103, after which the light beams are incident on the holographic optical element 104. The three diffracted light beams are transmitted by the holographic optical element 104, and are focused as a main spot and two sub-spots on the recording surface of the optical disk 100 by a light-focusing function of the objective lens 105.
The three laser light beams focused on the optical disk 100 are reflected as three returned light beams including an information signal having information recorded on the optical disk 100. The returned light beams are incident on the holographic optical element 104 upon passing through the objective lens 105. The holographic optical element 104 transmits the three returned light beams as + 1st order and - 1st order diffracted light beams. The + 1st order diffracted light beam is incident on a light receiving surface of the optical detector 106. The optical detector 106 outputs a reproducing signal, a focus error signal obtained by an astigmatism method, and a tracking error signal obtained by a three-beam method on the basis of the returned light beam (+ 1st order diffracted light beam) received.
In the above-mentioned conventional optical pickup apparatus, only one of the returned light beams transmitted as the + 1st order diffracted light beam and the - 1st order diffracted light beam by the holographic optical element 104, for example, the + 1st order diffracted light beam is introduced into the optical detector 106, and the other returned light beam, for example, the - 1st order diffracted light beam is not utilized. Therefore, a detecting current from the optical detector 106 which corresponds to the power of the laser light emitted from the semiconductor laser device 102 is small. In order to increase output power of an information detecting signal from the optical disk 100, therefore, an output of the semiconductor laser device 102 must be increased, or an output signal from the optical detector 106 must be amplified.
In a method of increasing the output of the semiconductor laser device 102, however, a load on the semiconductor laser device 102 may, in some cases, be excessively large. On the other hand, in a method of amplifying the detecting signal from the optical detector 106, the frequency band of the detecting signal is narrowed as the amplification factor is increased, thereby degrading the frequency characteristics.